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Tuesday, October 26, 2010

Peridot - Part 3: Weathering With Time

Hello Gemstone Fans:

I started this series of posts by saying that olivine was hardly a rare stone, what with it making up a substantial portion of the Earth's interior. And then, of course, I turned that on its head by saying that while olivine under the surface is common, olivine on the surface is hard to find. It is especially hard to find in the quality peridot needed to make cut gemstones, in fact.

The first reason such material is rare is that in order to get it to the surface, you need volcanism with magmas of the right composition, as I mentioned in my second post. The second reason is that olivine is not a particularly hard stone, and what peridot makes it to the surface weathers quickly. Weathering can take the form of physical weathering, which grinds down stones over time into small particles, and it can take the form of chemical weathering, where say water interacts with the stones and breaks down their crystalline structure.

On the Mohs hardness scale, peridot measures 6.5 to 7 (Diamond is the hardest at 10, with emeralds and sapphires at 9. Soft materials have low numbers, like talc at 1 and amber at 2.) This means there are many harder materials in the environment that can scratch peridot, assisting in its break down.

But this phenomenon does have some interesting effects. There are places on Earth where you can find stretches of sparkling green sand. Hawaii is one of these places, where there are small beaches of sand largely composed of grains of olivine.

How do such concentrations of olivine grains come to be? Hawaii is, of course, a volcanic island, with active volcanism today. There are places on the island where the black rocks are rich in olivine crystals. As those rocks weather and break down, the crystals of olivine trapped inside are liberated, and moved by wind and rain downslope. A green sand beach may form in any area where the nearby rocks are rich in olivine, and where the shape of the coastline can protect the sand. That way it collects there at approximately the same rate it is broken down or washed away.

I have traveled to the most well known of these beaches (shown in the top image) the green sand beach near South Point in Hawaii. There are other beaches with sands of interesting color, such as black, pink, white, and red. But on this beach I got to walk barefoot on piles of tiny, glittering green gemstones. I felt a little like a dragon with a hoard of gems ...

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